Fix descriptions for --enable-feature

[am97]

Currently, passing multiple --enable-feature doesn't work:

$ docker run --rm -it --entrypoint sh prom/alertmanager:v0.28.0
/alertmanager $ alertmanager --config.file=/etc/alertmanager/alertmanager.yml --enable-feature=auto-gomemlimit --enable-feature=auto-gomaxprocs
alertmanager: error: flag 'enable-feature' cannot be repeated, try --help

The intended syntax seems to be a single --enable-feature flag, with a comma-separated list of features as value. This is what is tested here:

alertmanager/featurecontrol/featurecontrol_test.go

Lines 40 to 44 in 3b06b97

	{
	name: "with both, valid and invalid feature flags",
	featureFlags: strings.Join([]string{FeatureReceiverNameInMetrics, "somethingbad"}, ","),
	err: errors.New("Unknown option 'somethingbad' for --enable-feature"),
	},

[grobinson-grafana]

I would prefer we put what it does first and the expected format second. Otherwise LGTM!